Rotor to Distributor Cap Clearance

R

Ron/IA

Well-known member
In the red mechanics handbook it states the clearance between the rotor and the contacts on the distributor cap should be set at .025". I looked in the Service Bulletins, and didn't find any information concerning said clearance. If others have found it in there, please direct me to the page.

An old article from The Restorer magazine states this should be set at .035", but a disclaimer in the article stated this dimension was used because the engine ran better at that setting. The article did not state if other clearances were tried. It also did not state the .025" dimension was the standard.

Another item in the article stated one should not try to bend the tab on the rotor, as there was a danger of it breaking. I can understand that, as the tab is pretty stiff metal.

Another statement in the article said one can build up the contacts on the distributor cap with solder, and file them to achieve the correct clearance.

My questions are:
1) Is there Ford documentation stating the .025" dimension is official?
2) Has any Fordbarners experimented with different clearances and found one dimension runs better than another?
3) Has anyone developed a method of bending the rotor tab without breaking it?
4) Has anyone tried the solder/file method on the distributor cap contacts?

Thanks for your replies?
 
Here are my experiences;

Ron/IA said:
My questions are:
1) Is there Ford documentation stating the .025" dimension is official?

Don't remember finding one

2) Has any Fordbarners experimented with different clearances and found one dimension runs better than another?

A larger gap allows for the voltage to build up to give a hotter spark (within limits) Keep in mind that it takes time to build up to the point where the spark will jump so it will effect timing, a little.)

3) Has anyone developed a method of bending the rotor tab without breaking it?

Yes

4) Has anyone tried the solder/file method on the distributor cap contacts?

Not Solder but file, I adjust the gap on the tab farthest away by bending and file the rest down to match.

Thanks for your replies?
Click to expand...
 
Mike - I might add I was aware a hotter spark (as you stated within limits) can develop due to a larger gap. With that, the article also stated the .035" gap mostly helped the engine run better at higher RPMs (road cruising speed).

Can you expound on the bending method of the rotor tab? Is there a way to bend it without putting stress on the bake-lite?
 
The coil that is used will make a difference in the air gap that can be uised. A weak replacement coil will produce colder spark and not jump a wider gap. Too wide of gap will cause the coil to run hotter and shorten coil life. The same applies to spark plug gap. I think that Les came up with the .025 gap . I run a minimum .035 air gap with all of mine, whether original or performance type coil is used. I set my points gap at .022 which advances the timing to the max without the possibility of starter kick back. That is of course if the rotor tip is set at the correct spot with no clock wise back lash, most probably aren't. Even wider rotor tip air gap can be used with the right performance coil. The only reason that I don't use wider gap than .035 is that i'm not racing. I'm just interested in hotter spark for a driver car with good reliability.
 
I saw in print a few years ago that Terry Burtz advised to set plug gap and rotor/ dist gap at same advised 35 for both. or whatever you run but they should be set the same. Have fun modelAtony Lafayette, LA
 
Purdy Swoft said:
The coil that is used will make a difference in the air gap that can be uised. A weak replacement coil will produce colder spark and not jump a wider gap. Too wide of gap will cause the coil to run hotter and shorten coil life. The same applies to spark plug gap. I think that Les came up with the .025 gap . I run a minimum .035 air gap with all of mine, whether original or performance type coil is used. I set my points gap at .022 which advances the timing to the max without the possibility of starter kick back. That is of course if the rotor tip is set at the correct spot with no clock wise back lash, most probably aren't. Even wider rotor tip air gap can be used with the right performance coil. The only reason that I don't use wider gap than .035 is that i'm not racing. I'm just interested in hotter spark for a driver car with good reliability.
Click to expand...

Hey Purdy,
Same practice/experimenting for me with one addition...I run plug gaps same size as rotor gap..i.e.- .035. Runs VERY well !

BTW..not sure about the 'idea' of shortening coil life, as I tried several and with no negative results to verify the idea. Of course, as you state, you must ensure proper coil is used in circuit to begin with.
 
Last edited:
hardtimes said:
Hey Purdy,
Same practice/experimenting for me with one addition...I run plug gaps same size as rotor gap..i.e.- .035. Runs VERY well !

BTW..not sure about the 'idea' of shortening coil life, as I tried several and with no negative results to verify the idea. Of course, as you state, you must ensure proper coil is used in circuit to begin with.
Click to expand...


Hey Rick,
I also match the plug gap to the rotor tip air gap . Right now I'm running gaps of .035 on all of mine. With my flame thrower coil, I could open up the gaps quite a bit more for even hotter spark. Mine seems to run super good with the gaps at .035. I don't get pinging or pre ignition. I do get instant starts and the plugs run clean. The greater the gaps that a coil has to jump at the plugs or at the rotor tip, the greater the strain on the coil and the hotter it will run. I had coils that I got at the tractor place fail quickly when I run the gap at .045 I think that a gap of .035 is good with any coil on the model A.
 
I did this measurement and found that the dist cap would move around thus you could measure .025 or less depending on where you move the dist . cap . The dist. cap I am using is a new cap from a major vendor and it is the best cap they sell . I plan to run some tape around the inside of the cap to a thickness which will cause the cap to be tight when installed then remeasure and adjust rotor as needed. Has anyone else encountered this problem,if so , what was your corrective fix .
 
I've also seen some issues with the sloppiness of the key on the rotor arm leading to some issues on badly made rotor arms.
 
Duffy1 said:
I did this measurement and found that the dist cap would move around thus you could measure .025 or less depending on where you move the dist . cap . The dist. cap I am using is a new cap from a major vendor and it is the best cap they sell . I plan to run some tape around the inside of the cap to a thickness which will cause the cap to be tight when installed then remeasure and adjust rotor as needed. Has anyone else encountered this problem,if so , what was your corrective fix .
Click to expand...

Yes, I've seen slop in the distributor bodies. I use an original on my car and it fits snug.
For customers I bought a body from Bratton's and another from Little Dearborn about 8 years ago and both fit snug, like they should.
 
Thanks for the response fellows . I also have a modern points nu-ex distributor which could be causing some of the problem . So a little off on the cap and a little off on the dist. and you have lots of slop. Has anyone had this problem with this dist .
 
Duffy1 said:
Thanks for the response fellows . I also have a modern points nu-ex distributor which could be causing some of the problem . So a little off on the cap and a little off on the dist. and you have lots of slop. Has anyone had this problem with this dist .
Click to expand...

By "CAP" if you mean the small round top, then a little side paly is no concern.

If the mean the main distributor body has side play, then it does matter, as the contacts to rotor gap changes.
 
OK, Dist. cap is now a tight fit . I filed the rotor ( new from major vendor ) so that I now have .025 gap . Before there was almost no gap. Increase in power is very noticeable. Good info here on the Ford Barn . Thanks Tom and everyone else for your input .
 
Duffy1 said:
OK, Dist. cap is now a tight fit . I filed the rotor ( new from major vendor ) so that I now have .025 gap . Before there was almost no gap. Increase in power is very noticeable. Good info here on the Ford Barn . Thanks Tom and everyone else for your input .
Click to expand...

Glad to help and thanks for letting us know what worked.
 
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